<?php

$numbers = array_map('intval', explode("\n", file_get_contents('input.txt')));

$current_index = 0;

$step = 25;

while (true) {
    for ($l = count($numbers) - $step; $current_index <= $l; $current_index++) {
        $number_to_test = $numbers[$current_index + $step];
        $numbers_to_addition = array_slice($numbers, $current_index, $step);
        foreach ($numbers_to_addition as $number) {
            if (in_array($number_to_test - $number, $numbers_to_addition, true)) {
                $current_index++;
                break 2;
            }
        }
        echo $number_to_test;
        exit;
    }
}